The molecular evolution of primate DNA double-strand break repair genes will be studied. These genes are important for genome stability, and many cancer syndromes have been linked to mutations in these genes. Interestingly, many members of this gene family bear signatures of recurrent positive selection. The hypothesis being tested is that these signatures result from the long-term co-evolution between these DNA repair genes and the pervasive, parasitic retroviruses and retrotransposons with which primates have evolved. This may have influenced primate DNA repair genes with respect to both polymorphic and fixed genetic variation. In this proposed research, large primate sequence datasets will be generated for genes involved in human double-strand break repair, and these will be tested for signatures of recurrent positive selection. Retroviral infection assays will then be used to test whether adaptive sequence change in these DNA repair genes has resulted in altered susceptibility to infection. The consequences of this sequence change to cellular DNA repair will also be explored. Preliminary data in a cell culture model system shows that one of the most well-known cancer susceptibility genes, BRCA1, may be evolving under the dual selective pressures of DNA repair fidelity and retroviral resistance. Based on this, DNA repair gene evolution can have a profound effect on cancer, and for this reason, it is of great importance that the evolutionary forces that define the evolution of these genes be elucidated. This proposed research is central to the overarching goal of understanding how genetic parasites, through the selective pressures that they exert, shape the sequence of human genes with which they interact.